The best translation would be “keyboard warrior” or “keyboard warriors” as explained here: https://chinadigitaltimes.net/space/Keyboard_warrior “Keyboard man” is an awkward translation that doesn’t really make sense. “Cyberbully” makes more sense than “keyboard man”, but if you’re referring to a person who acts tough online rather than a person who acts mean towards others online “keyboard warrior” is more accurate than “cyberbully”. If your intent is to talk about someone who acts outraged online but doesn’t do anything about it in person none of these terms work perfectly. Maybe something like “slacktivist” would do in that case.

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Hi, Nanren! Thanks for your comments. You are so kind!😆 And one of your guesses is right. 键盘侠 means that some people are timid in real life, but they publish "sense of personal justice" and "personal comments" on the Internet. Some people refer it to keyboard man. But I think it sounds like translate it directly, that's why I ask this question. But I don't understand what does real-life grey man mean? Does it mean 现实中是透明人？ And I also confused about ''Online crusader but real-life nobody''? Could you explain them to me? Thanks for your reading and explaination again!💐

Without the explanation given below “keyboard warrior” has no obvious meaning to me. (Native speaker of New Zealand English.) cyber bully - understood, but not meaning the same thing. From the link provided. Pejorative for a person quick to fight for justice on the Internet without displaying the same commitment in the real world. (This definition is confusing in that it suggests a separation between "online" and "the real world".) The definition given above is the best translation that I can see. That is, it has not succinct English translation in normal English. So, feisty online, but a wimp in real life? Online defender but real-life grey man? Online crusader but real-life nobody? . Among cyber-communities, a specialist jargon does tend to develop. It may be a term exists for a sub-section of society.